A good point that I think you could discuss is the role of the Australian Government in the book as an opposing pressure (or barrier) to the characters’ sense of belonging.
Their family has been fractured as a result of government policies and this is clear through the segregation of men and family in the workplace. (there are other examples, such as the Gov rejecting Hora’s application to adopt Mitru’s daughters)
So, from this, you could talk about the context of the book; specifically the shift in attitudes towards migrants and the White Australia Policy. Although the Policy was not abolished until a bit later (1966?), there was nonetheless a relaxation of restrictions. Point out how Government attitudes and policies both shape and reflect the way we look at people, thus linking back to your question on the government as an opposing pressure that the characters struggle against.
Mental illness as an opposing pressure can be talked about quite comprehensively. Due to Christina’s psychological barrier, she is judged and subsequently alienated even further. In answering the question though, I guess you could say that with her mental disorder as an opposing pressure, she struggles with it to such an extent that it disconnects her from everyone else, generating a sense of not-belonging. So pretty much she fails to overcome it and therefore does not achieve a sense of belonging.
You can also discuss the fact that it is through the struggling of these opposing pressures that forges a sense of belonging. Eg. As a result of Christina neglecting Raimond, the relationship between Romulus and Raimond is consequently solidified.
Just some ideas. Good luck